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For decades, there were hints that Greenland sharks may be exceptionally long-lived.

But the sharks, which inhabit the coldest, deepest waters of the North Atlantic, are a cryptic animal. Little is known about their breeding, their feeding, their population size. And efforts to accurately calculate the Greenland shark’s lifespan were flummoxed: none of the usual methods for determining the age of vertebrate animals could be used.

Of the 28 Greenland sharks the scientists examined, the likeliest age of the largest was 392 years old. The majority were in their 100s and 200s, with only three, the tiniest, younger than 60.

The dating methods involve a lot of uncertainty, so the oldest shark could be as young as 272 or as old as 512. But even if the lowest age is accurate, Greenland sharks would be the longest-lived known vertebrate.

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“I’m not sure how exact it is, but I think, in general — and I think the paper was fair about being honest about that — I do think they’ve provided solid evidence that these sharks are very old, centuries old,” said Aaron Fisk, a professor at the University of Windsor and Canada Research Chair in trophic ecology, who studies Greenland sharks. “It’s amazing to think a vertebrate can live that long.”

Julius Nielsen, first author on the Science paper, was a student aboard a Greenland Institute of Natural Resources research vessel when he first laid eyes on one of the sharks, caught accidentally — as bycatch — during a scientific survey. Adult females grow to between four and five metres and this shark turned out to be one of the largest ever reported.

“For everyone, it was an amazing experience to see this big animal,” said Nielsen, now a doctoral student in marine biology at the University of Copenhagen. And when he examined the literature, “it was really surprising to realize there were so many mysteries associated with the fundamental biology of this animal.”

Decades-old research indicated that Greenland sharks grow extremely slowly: when researchers caught one shark twice more than a decade apart, they found it had grown less than a centimetre per year. Again, adults can grow to 500 centimetres. You do the math.

In bony fish, researchers can count layers in bones known as “otoliths,” or ear stones, to help age the animals. Aging any shark is difficult because their skeletons are made of cartilage, but can be achieved in some species by analyzing growth rings in vertebrae.

None of that works for Greenland sharks.

“This was just like — wow, it’s amazing they could potentially be extremely old, said Nielsen. But “because normal determination methods do not apply on the Greenland shark, it has just remained a mystery for so many years.”

Several months after his experience aboard the research vessel, Nielsen was sitting in on a lecture when another researcher raised the idea of dating the sharks’ eye lenses, a method that had worked with bowhead whales — another very long-lived species.

The centre of the eye lens develops before birth, with successive layers laid down over time. After peeling back the layers like a microscopic onion, the nucleus, made of unique crystalline proteins, can be radiocarbon dated.

Nielsen realized that the scientific surveys in Greenland, where the sharks were already being collected as bycatch, provided specimens to test this hypothesis.

With left eye lenses from 28 sharks collected over three years, Nielsen and his colleagues first analyzed the nuclei for a “bomb pulse:” distinctive carbon isotope signatures produced by thermonuclear weapon tests in the 1950s and absorbed in marine species by the early 1960s.

“The bomb pulse only affected the small sharks — that was a big surprise to me,” said Nielsen. Just three of the sharks were affected. “We realized immediately, Jesus Christ, all of our sharks are extremely old — they’re much older than we expected.”

For the “pre-bomb” sharks, the researchers applied typical radiocarbon dating techniques to reveal a range of potential ages for each animal, with the likeliest age at the midpoint. The two largest, at 483 and 502 centimetres, were estimated to be 335 and 392 years old.

Researchers that study human longevity are interested in studying the genomes of exceptionally long-lived species, including the bowhead whale. But the main conclusion of the Science study relates to the sharks themselves.

The new research suggests that Greenland sharks only reach sexual maturity at about 150 years old. They appear to be plentiful now. But this incredibly late reproductive age could leave the species vulnerable to human impacts.

“We are looking more and more to the Arctic to fish, and there are some very large, very valuable fish stocks up there,” said Fisk.

“This kind of information showing that these animals are so long-lived really needs to be considered as we figure out how we’re going to manage these arctic ecosystems.”

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